As one skilled in the gas turbine technology recognizes, the efficiency of the engine is enhanced by operating the turbine at a higher temperature and by increasing the turbine's pressure ratio. Another feature that contributes to the efficacy of the engine is the ability to cool the turbine with a lesser amount of cooling air. The problem that prevents the turbine from being operated at higher temperatures is the limitation of the structural integrity of the turbine component parts that are jeopardized in its high temperature, hostile environment. Scientist and engineers have attempted to combat the structural integrity problem by utilizing internal cooling and selecting high temperature resistance materials. The problem associated with internal cooling is twofold. One, the cooling air that is utilized for the cooling comes from the compressor that has already expended energy to pressurize this air and the spent air in the turbine cooling process in essence is a deficit in engine efficiency. The second problem is that the cooling is through cooling passages and holes that are in the turbine blade which, obviously, adversely affects the blade's structural prowess. Because of the tortuous path that is presented to the cooling air, the pressure drop that is a consequence thereof, requires higher pressure and more air to perform the cooling that would otherwise take a lesser amount of air given the path becomes less tortuous to the cooling air. While there are materials that are available and can operate at a higher temperature that is heretofore been used, the problem is how to harness these materials so that they can be used efficaciously in the turbine environment.
To better appreciate these problems it would be worthy of note to recognize that traditional blade cooling approaches include the use of cast nickel based alloys with load-bearing walls that are cooled with radial flow channels and re-supply holes in conjunction with film discharge cooling holes. Example of these types of blades are exemplified by the following patents that are incorporated herein by reference.                U.S. Pat. No. 4,257,737 granted to D. E. Andress et al on Mar. 24, 1981 entitled “Cooled Rotor Blade”;        U.S. Pat. No. 4,753,575 granted to J. L. Levengood et al on Jun. 28, 1988 entitled “Airfoil with Nested Cooling Channels”;        U.S. Pat. No. 5,476,364 granted to R. J. Kildea on Dec. 19, 1995 entitled “Tip Seal and Anti-Contamination for Turbine Blades”; and        U.S. Pat. No. 5,700,131 granted to Hall et al on Dec. 23, 1997 entitled “Cooled Turbine Blades for a Gas Turbine Engine”.        
Also well known by those skilled in this technology is that the engine's efficiency increases as the pressure ratio of the turbine increases and the weight of the turbine decreases. Needless to say these parameters have limitations. Increasing the speed of the turbine also increases the airfoil loadings and, of course, satisfactory operation of the turbine is to stay within given airfoil loadings. The airfoil loadings are governed by cross sectional area of the airfoil of the turbine multiplied by the velocity of the tip of the turbine squared. Obviously, the rotational speed of the turbine has a significant impact on the loadings.
The spar/shell construction contemplated by this invention affords the turbine engine designer the option of reducing the amount of cooling air that is required in any given engine design and in addition, allowing the designer to fabricate the shell from exotic high temperature materials that heretofore could not be cast or forged to define the surface profile of the airfoil section. In other words, by virtue of this invention, the skin can be made from Niobium or Molybdenum or their alloys, where the shape is formed by a well known electric discharge process (EDM) or a wire EDM process. In addition, because of the efficacious cooling scheme of this invention, the shell portion could be made from ceramics, or more conventional materials and still present an advantage to the designer because a lesser amount of cooling air would be required.